Thanks a lot @gniibe for this change.
I do understand and share your concerns, nevertheless are there, in my opinion valid reasons to be able to have a backup or duplicate, especially on the same or similar media type.
Consider for example giving multiple devices a chance of common interaction, using the keys for backup encryption etc. - I think there are several possible use-cases which can benefit from this.

Hello again - can I ask about the status? Or should I consider this as a no-fix? Anything I can assist with?

I would have thought this is a logical usage for gpg cards - seems this is harder to achieve as I thought.

That's exactly the point: I do want one common encryption key between the two cards: So I can distinguish between the two, but en-/decrypt with both.
One is on the GnuPG SmartCard, the other on a YubiKey - output --card-status (some things xxx'ed out) :